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The Forefront of Space Science

Lifetime of Interstellar Dust Explored by AKARI Satellite
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Interstellar dust

A myriad of minute solid particles are drifting between stars in outer space. They are called “interstellar dust.” The size of the smallest dust is only a few nanometers. It is believed that the dust plays an important role as the raw material for solid matter of planetesimals in the solar system, for example, and as building blocks in the formation of organic matter, or the origin of life, since the dust undergoes complex chemical processes in various astronomical phenomena and interstellar environments in outer space. Therefore, our understanding of the birth and evolution of dust in outer space is indispensable to learn how the current chemically and physically rich universe was formed, and is also an important theme for todayís astronomy.

Infrared emitted by dust

What then should we do to understand the nature of the dust? First, we need to understand the light emitted from the dust. Interstellar dust absorbs visible and ultraviolet light radiated mainly from stars and is thus heated to tens to hundreds K (Kelvin) in absolute temperature. As a result, the dust itself radiates infrared. The spectrum of infrared emitted from the dust varies according to the dustís material, even at the same temperature. Therefore, by analyzing in detail spectra from the dust observed by infrared, we can identify both the type of material and the type of physical environment. The satellite is ideal for this infrared observation because its observation is not disturbed by thermal radiation or absorption of the earthís atmosphere. For this reason, we are conducting observations by the infrared astronomical satellite AKARI in order to elucidate the entire life of interstellar dust in outer space. In this article I will introduce some of the research results based on AKARIís observations.

Site of dustís birth around a supernova observed by AKARI

Interstellar dust contains a variety of elements. For example, carbonaceous dust is composed of graphite or diamond powders while silicon dust contains glass, quartz or sapphire powders. These elements that are heavier than hydrogen or helium are synthesized by nuclear reaction occurring in stars during their evolution. Dust is believed to form through the following processes: when stars die, they release gases containing chemically affluent elements to outer space and, as these gases cool, dust is produced. There is little observational evidence, however, on what kind of dust is created from which type of starís death following evolution. Many mysteries remain unsolved about the dustís birth process.

Incidentally, supernova explosions are an astronomical event that stars of nearly eight times or more the Solar mass come to the end of their lives and scatter gases containing synthesized elements to outer space. Only hydrogen and helium existed at the beginning of the universe. It is thought that supernova explosions played an important role in the universeís transformation to its current state, with a wide range of solid matter, such as current planets and organisms, as well as chemically rich environments. In particular, it is supposed that these explosions played a critical role in the chemical evolution in the early universe because the evolutionary life of heavy stars capable of causing supernova explosions is short (about several million years) and, therefore, the stars can efficiently provide chemically rich matter to outer space. If the formation process of dust around supernovae could be unraveled observationally, we would be able to learn how high-mass stars contribute to the birth of dust drifting in space, in addition to the contribution of low- to medium-mass stars that come to an end without supernova explosions. Thus, we can obtain important information about the origin of interstellar dust in the early universe.

Figure 1
Figure 1a. Composite image in three false colors of the supernova 2006jc about a half year after the explosion and the mother galaxy UGC4904 photographed by near- to mid-infrared camera onboard AKARI. The image was synthesized from observed data of 3μm (blue), 7μm (green) and 11μm (red). Color of general stars is blue while the supernova 2006jc shines in red at almost the same level as the galaxy. Thermal radiation from dust born around the dying supernova is observed.
Figure 1b. Photometric and spectroscopic data of near- to mid-infrared of the supernova 2006jc about a half year after the supernova explosion. These data were retrieved by AKARI and the MAGNUM telescope. The gray-colored solid line in the figure shows the model calculation result of dust thermal radiation, which most closely reproduces the observed data. This reveals that relatively low-temperature carbonaceous dust at about 50 deg. C (green solid line) exists in addition to the high-temperature carbonaceous dust at about 500 deg. C (red dotted line).

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